Multi-user multiple-input multiple-output (MU-MIMO) systems with a large number of base-station antennas may serve as high throughput communications for emerging wireless deployments. By spatially multiplexing signals, the base-station antenna array may serve many separate user terminals using the same time-frequency resource. This spatial resource sharing policy may serve as an alternative to costly spectrum licensing. The policy may also avoid the costly procurement of additional base stations used in conventional cell-shrinking strategies.
While the benefits of spatial multiplexing may be fully realized when the number of base-station antennas is equal to the number of scheduled user terminals, MU-MIMO systems with an excessively large number of antennas, also known as “Massive MIMO” may also provide additional benefits. Massive MIMO can increase the system's capacity while simultaneously improving the radiated energy efficiency via energy-focusing. Massive MIMO systems can also be integrated with inexpensive, low-power components by replacing expensive high-power linear amplifiers with low-power counterparts (e.g., mW rather than W). Massive MIMO can also simplify the multiple-access (MAC) layer by scheduling users on the entire band without the need for feedback. As the number of antennas in a MIMO cell grows larger, uncorrelated noise and small-scale fading may be mostly eliminated and the required transmitted energy-per-bit may be significantly reduced.
Unfortunately, massive MIMO implementations present some very challenging aspects, including, e.g.: antenna design, pilot contamination, intercell interference management, and hardware impairments. Signal processing may resolve some of these complications. Zero-forcing (ZF) beamforming, for example, is a signal processing technique, which has been shown to yield very high spectral efficiencies under favorable conditions. ZF precoding eliminates the interference between the user data streams in the downlink MIMO channel. The MIMO channel matrix includes channel state information (CSI) at the base-station acquired during a pilot transmission phase in the uplink from the user terminals. Once the CSI is acquired, the ZF decoupling operation can be realized via complex matrix inversions on the MIMO channel matrix at the base-station.
Unfortunately, matrix inversion can become computationally expensive for MIMO systems with a very large number of user terminals since the inversion incurs cubic computational complexity in the number of users. Accordingly, there exists a need for improved ZF precoder operations that allow for scalable massive MIMO system design.
Those skilled in the art will appreciate that the logic illustrated in various of the figures including the flow diagrams discussed herein, may be altered in a variety of ways. For example, the order of the logic may be rearranged, substeps may be performed in parallel, illustrated logic may be omitted, other logic may be included, etc. Those skilled in the art will appreciate that actual data structures used to store information may differ from what is shown, in that they, for example, may be organized in a different manner; may contain more or less information than shown; may be compressed and/or encrypted; etc.
The headings provided herein are for convenience only and do not necessarily affect the scope or meaning of the claimed embodiments. Further, the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be expanded or reduced to help improve the understanding of the embodiments. Similarly, some components and/or operations may be separated into different blocks or combined into a single block for the purposes of discussion of some of the embodiments. Moreover, while the various embodiments are amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the particular embodiments described. On the contrary, the embodiments are intended to cover all modifications, equivalents, and alternatives falling within the scope of the disclosed embodiments as defined by the appended claims.